With lengthened intervals of oil changes, need of energy-saving, use of high performance machines, and down-sizing of machines, demand for the performance of lubricating oils has become severe. In particular, lubricating oils with a high thermal stability and a high oxidation resistivity have been strongly sought. In the situation where there has been increasing public concern about global environmental pollution, such as depletion of ozone layer caused by freon, the earth warming due to excess carbon dioxide and methane, destruction of forests by sulfur dioxide and NO.sub.x in exhaust fumes, and pollution of soil and lakes due to chemical leakage, environmental protective measures have been strongly sought also in the field of lubricating oils.
In order to meet the requirements for high thermal stability and oxidation resistivity, ethers, such as polyalkylene glycols, and esters, such as aliphatic diesters and hindered esters, have been developed and used for engine oils, hydraulic oils, grease base oils, gear oils, rolling oils, precision instrument oils, etc.
However, ethers, such as polyalkylene glycols, have a higher polarity than conventionally used mineral oils to cause the following drawbacks: (1) high hygroscopicity, and (2) insolubility of additives which are conventionally used for lubricating oils. An ether synthesized from an alkyl halide and a polyol, which has a lower polarity than polyalkylene glycols, is described in Japanese Patent Examined Publication No. 1-29240. This ether also has a problem that a trace of residual halogen in the ether product may impair thermal stability and oxidation resistivity.
Esters also are not free from drawbacks. That is, carboxylic acid formed by hydrolysis of esters erodes metals, and esters tend to impair the effects of oiliness improvers and extreme pressure additives, though esters have a merit to reduce friction because they are well adsorbed onto metals.
Thus, synthetic lubricating oils with adequately low polarity, high thermal stability, high oxidation resistivity, no formation of carboxylic acids due to hydrolysis and low hygroscopicity are in high demand.
Recently, the use of dichlorodifluoromethane (CFC12) for refrigerators and car air conditioners is restricted, and will be legally banned at the end of 1995 in order to protect the ozone layer from the viewpoint of the environment of the earth, and also the use of chlorodifluoromethane (HCFC22) for room air conditioners is about to be legally regulated. Thus, hydrofluorocarbons which do not destroy the ozone layer, such as 1,1,1,2-tetrafluoroethane (HFC 134a), difluoromethane (HFC32), and pentafluoroethane (HFC125), have been developed as substitutes for CFC12 or HCFC22.
However, since the polarity of hydrofluorocarbons is higher than that of CFC12 or HCFC22, the use of conventional lubricating oils, such as naphthene mineral oils, poly-.alpha.-olefins and alkylbenzenes, causes two-phase separation of the working fluid at low temperatures. This is due to poor compatibility between the conventional lubricating oils and the hydrofluorocarbons. Two-phase separation hampers oil return, which in turn interferes with heat transfer due to deposition of a thick oil film around the condenser and evaporator used as heat exchangers. It can also cause significant failures, such as poor lubrication and foaming upon starting operation. Therefore, the conventional refrigeration oils cannot be used as refrigeration oils under these new refrigerant atmospheres. With this being the situation, lubricating oils with good compatibility with hydrofluorocarbons have been sought.
As for lubricity, CFC12 and HCFC22 generate hydrogen chloride upon its partial decomposition. The hydrogen chloride thus formed reacts with the friction surface to form a coating of chlorides, thereby improving the lubricity. On the other hand, hydrofluorocarbons containing no chlorine atoms are not expected to have such an effect; therefore, the refrigeration oils used in combination with hydrofluorocarbons are required to have a further excellent lubricity when compared to the conventional refrigeration oils.
In addition, the refrigeration oils used in combination with hydrofluorocarbons have to have good thermal stability in the presence of hydrofluorocarbons.
Moreover, in the compression-type refrigerating machines for electric refrigerators and air conditioners, since organic materials are used for motor components such as insulators and enameled wires, the working fluid comprising a hydrofluorocarbon and a refrigeration oil is required to have no adverse effects on the organic materials and also have a good insulating property.
As refrigeration oils which can be used in combination with hydrofluorocarbons such as 1,1,1,2-tetrafluoroethane (HFC134a), polyalkylene glycol compounds are disclosed in U.S. Pat. No. 4,755,316 (Japanese Patent Unexamined Application No. 2-502385), Japanese Patent Laid-Open No. 3-14894 (European Patent No. 377122), and Japanese Patent Laid-Open No. 2-182780 (WO90/05172).
Since polyalkylene glycol compounds have a higher polarity than naphthene mineral oils, their compatibility with HFC134a at low temperatures is certainly good. However, polyalkylene glycol compounds have a problem to cause phase separation as the temperature increases, as mentioned in U.S. Pat. No. 4,755,316. There are also several other problems about polyalkylene glycol compounds. A poor insulating property is one of the problems. Due to this significant problem, polyalkylene glycol compounds cannot be used for refrigerating devices of electric refrigerators and air conditioners where a motor is incorporated in a compressor. Therefore, applications of polyalkylene glycol compounds are proposed for car air conditioners where their poor insulating property does not cause any problems. High hygroscopicity is another significant problem of polyalkylene glycol compounds. Water absorbed by the compounds causes thermal instability in the presence of HFC134a and hydrolysis of organic materials such as PET films. Insulating property can be improved and hygroscopicity is decreased by reducing the number of ether bonds per unit weight of polyalkylene glycol compounds, but the compatibility with hydrofluorocarbons becomes poor. Thus, ether compounds like polyalkylene glycol compounds cannot have both good compatibility with hydrofluorocarbons and good insulating property/decreased hygroscopicity at the same time.
In order to solve the above problems of polyalkylene glycol compounds, such as poor insulating property and high hygroscopicity, ester compounds and carbonate compounds have been developed. For example, the following compounds are disclosed as refrigeration oils which can be used in combination with 1,1,1,2-tetrafluoroethane (HFC134a): mixed oils of polyalkylene glycol compounds and ester oils are disclosed in U.S. Pat. No. 4,851,144 (corresponding to Japanese Patent Laid-Open No. 2-276894) and Japanese Patent Laid-Open No. 2-158693; ester oils are disclosed in Japanese Patent Unexamined Application No. 3-505602 (WO90/12849) and Japanese Patent Laid-Open Nos. 3-128991, 3-128992, 3-88892, and 3-179091; and carbonate oils are disclosed in Japanese Patent Laid-Open Nos. 2-132178 and 3-149295, and European Patent No. 421,298.
Ester compounds and carbonate compounds show good compatibility with hydrofluorocarbons and high thermal stability in the presence of hydrofluorocarbons. Also, these compounds have markedly better insulating properties and much lower hygroscopicity than polyalkylene glycol compounds. However, compared with the conventional CFC12-mineral oil working fluid system, both freon and oil tend to have a high polarity in the hydrofluorocarbon-ester oil system or hydrofluorocarbon-carbonate oil system, and the systems become highly hygroscopic. Therefore, esters tend to be hydrolyzed to form carboxylic acids, and the formed carboxylic acids may in turn erode metals and cause to wear down the metals. Also, in the case of using a carbonate oil, there arises a problem that a non-condensable carbon dioxide gas is generated owing to hydrolysis of the carbonate oil to cause low refrigerating capacity.
In particular, in the case of room air conditioners, it is common practice to fill an air conditioner with a refrigerant upon installation. Therefore, unlike refrigerating machines for which filling of refrigerant is carried out in a factory, it is almost impossible to prevent a working fluid of room air conditioners from being contaminated with water. Therefore, there has been a concern about the reliability of the hydrofluorocarbon-ester oil system and hydrofluorocarbon-carbonate oil system, when these systems are used in room air conditioners.
WO93/24435 discloses a polyvinyl ether compound as a polyether compound which is free from the problem of poor insulating property of polyalkylene glycols. This polyvinyl ether compound is prepared by polymerization of vinyl ether monomers and subsequent hydrogenation. It is described that the compound has a good compatibility with hydrofluorocarbons and good insulating property. However, since the polyvinyl ether compound is synthesized by polymerization, it shows molecular weight distribution. Therefore, a part of high molecular weight polymers sometimes causes plugged capillaries and worsens the compatibility of the compound with hydrofluorocarbon. Also, the compound requires complicated post-treatment and cannot always be obtained in a high yield because vinyl ether monomers, the starting materials of the polyvinyl ether compound, are not stable substances. In particular, the yield of those with low degree of polymerization (around 6) is low. Some vinyl ether monomers of certain structures cannot be easily obtained, and are, therefore, very expensive.
Alternatively, Japanese Patent Laid-Open Nos. 4-320498 and 6-57243 disclose the use of cyclic ketals and cyclic acetals for a working fluid composition for a refrigerating machine. In the former publication, it is disclosed that ketals or acetals prepared from monohydric or dihydric alcohols and ketones or aldehydes are used in combination with a synthetic lubricating oil of ester or polyalkylene glycol type. In the latter publication, ester or carbonate derivatives of glycerol, trimethylolethane, trimethylolpropane and pentaerythritol, which have ketal or acetal groups in the molecules, are disclosed. The acetals and ketals disclosed in the former publication have drawbacks of small molecular weight and low boiling and flashing points. The acetals and ketals disclosed in the latter publication have drawbacks of high cost because their production has two or more reaction steps. Some acetals or ketals in the latter publication also have a drawback of difficulty in producing them at high purity. Also, acetals and ketals disclosed in the latter publication have 2 or more ether bonds in a molecule, resulting in unsatisfactory improvement of insulating property.
The refrigerant-oil systems developed so far have some problems as mentioned above. The hydrofluorocarbon-polyalkylene glycol oil system has problems in hygroscopicity and insulating property; and the hydrofluorocarbon-ester oil system and the hydrofluorocarbon-carbonate oil system have problems of poor resistance against hydrolysis. Both of these systems are unsatisfactory as a working fluid composition for a refrigerating machine, because they have higher hygroscopicity as compared with the conventional CFC12-mineral oil system and cause thermal instability, deterioration of organic materials, and corrosion and wear of metals. Polyvinyl ether compounds show a molecular weight distribution, and therefore partially contain molecules with high molecular weights, which cause to lower the compatibility with hydrofluorocarbons. Polyvinyl ether compounds also have drawbacks of limited availability of the starting materials and high cost. Ketals and acetals reported so far have drawbacks of low molecular weight and high cost.
It is common practice to form cyclic acetals by using acetone as a ketone for the purpose of protection in the field of sugar and polyol synthesis. Recently, 1,3-dioxolan synthesized from formaldehyde and ethylene glycol is on the market as an organic solvent.
As for cyclic acetals formed from sorbitol and benzaldehyde, dibenzylidenesorbitol is known as a gelling agent. Also, cyclic acetals for med from an unsaturated aldehyde and sorbitol are utilized as a starting material of polymers.
Also, cyclic acetals formed between sorbitol and acetone, formaldehyde, acetaldehyde, or butyraldehyde are known. However, cyclic acetals having longer, linear or branched chains have not yet been known. Cyclic acetals formed between sorbitol and isobutyraldehyde are not known.
Cyclic acetals are used as organic solvents as mentioned above, as solvents for ink and paint, as additives or gelling agents, as starting materials for polymers, as absorbents for absorption refrigerating machines (Japanese Patent Laid-Open No. 59-25892), or as dehydrating agents for refrigeration oils (Japanese Patent Laid-Open Nos. 4-320498 and 6-57243).
Cyclic ketals have been formed between hexahydric alcohols, such as sorbitol and mannitol, and acetone for the purposes of protection of a compound and identification of structures. However, such cyclic ketals have high melting points and can hardly be used as polar oils, synthetic lubricating oils and refrigeration oils which are required to be in a liquid state around room temperature. Although cyclic ketals having unsaturated alkyl chains have lower melting points, they have drawbacks in stability. Those having aromatic rings such as a phenyl ring are to be gelatinized, and therefore, unsuitable as polar oils, synthetic lubricating oils, and refrigeration oils. On the other hand, cyclic acetals formed from lower hydric alcohols, such as ethylene glycol, have low viscosity and, therefore, unsuitable for the use in synthetic lubricating oils and refrigeration oils.